On Friday, August 09, 2013 04:16:56 PM Toshi Kani wrote:

On Fri, 2013-08-09 at 15:28 +0800, Tang Chen wrote:

quoted

On 08/07/2013 12:56 AM, Toshi Kani wrote:

quoted

On Tue, 2013-08-06 at 19:11 +0900, Yasuaki Ishimatsu wrote:

quoted

try_offline_node() checks that all cpus related with removed node have been
removed by using cpu_present_bits. If all cpus related with removed node have
been removed, try_offline_node() clears the node information.

But try_offline_node() called from acpi_processor_remove() never clears
the node information. For disabling cpu_present_bits, acpi_unmap_lsapic()
need be called. But acpi_unmap_lsapic() is called after try_offline_node()
runs. So when try_offline_node() runs, the cpu's cpu_present_bits is always
set.

This patch moves try_offline_node() after acpi_unmap_lsapic().

Signed-off-by: Yasuaki Ishimatsu<redacted>

The change looks good to me.

Acked-by: Toshi Kani<redacted>

BTW, do you know why try_offline_node() has to use stop_machine()?

try_offline_node() is used to check if the node could be hot-removed
after each memory or cpu hot-remove operation.

In memory hot-remove path, we have lock_memory_hotplug() to series all
the memory hot-remove options.

But when doing cpu hot-remove,

acpi_processor_remove()
  |->try_offline_node()

There is no lock to protect it. I think, when we are going to hot-remove
a node, others should not do any memory or cpu hotplug operation. In memory
hotplug path, we have lock_memory_hotplug(). But in cpu hotplug path, I
didn't find any lock. So we used stop_machine() to call check_cpu_on_node().
If we find any cpu still present, we return and do not remove the node.

CPU/Memory hotplug operations and sysfs eject are serialized with
acpi_os_hotplug_execute().  CPU online/offline is protected by
cpu_hotplug_[begin|done]() and [get|put]_online_cpus().  But, yes,
online/offline and hotplug operations are not serialized.  I tried to
serialize them before, but that framework was not well received.

What about lock_device_hotplug()?  It is taken by both online/offline and
the ACPI hotplug code, isn't it?

Anyway, it looks to me that cpu_up()->mem_online_node() path can race
with try_offline_node().

It can in principle, but I'm not sure if there's a way to trigger that
race.  Do you have an example?

Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.